For various reasons, polymer melts have to be modified with additives. For example, such melts are substantially transparent because the homogeneous structure of the synthetic polymers does not permit refraction or reflection of light. Therefore, for applications, pigments, such as TiO.sub.2, ZnS, or carbon black, are added to polymer melts. It is desirable to rid the melt before spinning of the unattractive, greasy luster which has a disturbing esthetic effect, especially when the fibers are to be used for clothing. The elimination of the luster takes place by adding a relatively small amount of a whitening pigment, usually titanium dioxide. The modification of polymer melts by the use of additives for other purposes is also known; e.g. for producing antipilling fibers or for the optical brightening of polymers with e.g. a yellow tinge.
With the increasing output of continuous polycondensation plants, only production lines with a high degree of flexibility are economical. Often, the parallel production of different types of products, such as fiber and granules, must be possible at the same time on a single production line. In such cases, the change-over to different products, such as for example, delustered, lustered, or pigmented, should be effected within the shortest time and, as far as possible, with no loss. These requirements are met only inadequately by the methods available heretofore.
It is well known that melts can be modified with the help of masterbatch products. This principle is described in German Offenlegungsschrift 16 04 368. A melt from masterbatch granules and containing a high percentage of additive is introduced into the polymer melt in a defined ratio. Although this process offers high flexibility with respect to change-overs of finish, it has substantial disadvantages. First, the masterbatch granules must be dried before melting; second, during remelting they undergo additional thermal and mechanical loading; and third, there are additional handling costs. Another disadvantage is the risk of external soiling of the granules. In the case of in-house masterbatch production, there is the added drawback that polymer granules have to be taken from the production process. If the masterbatch granules are obtained elsewhere, there are likely to be differences between the polymers used which have an adverse effect on the properties of the final product.
A variant of the above process is described in Chemical Engineering Progress 78 (1982) 1, pages 62-64. Instead of introducing the additive into the polymer melt downstream, both the polymer granules and the preferably reactive additives are introduced into the first extruder zone. The disadvantages of this process are, on the one hand, the risk of compacting of the additive between the granules and, on the other hand, the high shearing stresses which are necessary for homogenization and dispersion. The latter result in a large, uncontrollable reduction in the viscosity of the polymer.
A further state of the art process is described in Chemiefasern und Textilindustrie 1 (1986), pages 24 to 29. A partial stream is branched off from the main stream emerging from the polycondensation finisher and the additive is incorporated into the melt downstream. This additive-charged melt is recycled into the main melt stream. This process also has a high degree of flexibility in additive change-overs, but also several major disadvantages. One is that, in spite of evacuation, the volatile constituents contained in the polymer melt, such as glycol, water, or oligomers, due to vaporization thereof, escape through the filling funnel during the feeding-in of the additives. This results in reduced free-flowability of the additive and increases lump formation. Additive agglomerates can no longer be incorporated homogeneously into the melt and, during the spinning process, they cause clogging of the filters as well as weak points in the filaments. In addition, continuous, uniform metering of the additive is impaired by the thermal convection resulting from the hot melt and barrel, which keeps the additive particles, usually fed in free falls, in suspension at the feeding point. A further disadvantage is that the extruder opening for the feeding of the additive must be kept small in order to reduce the risk of melt escaping at this point. Consequently, the amount of additive is also limited.